Hyperthermia constitutes an effective adjuvant treatment for malignant tumors which are refractory to conventional therapy with surgery, radiation or chemotherapy. Hyperthermia can be administered to a patient either by an externally applied electromagnetic or ultrasound source, or internally by an interstitial heating technique. Implantable microwave antenna heating has proven the most popular of the three present interstitial heating modalities.
However, major problems arise with the use of conventional half-wavelength dipole antennas which severely limit the applicability and effectiveness of interstitial microwave hyperthermia. Such problems include: variability of heating profiles when the antenna is inserted to different depths in the tissue to be treated, restricted range of possible heating lengths for a given microwave frequency, and the presence of a so-called "cold region" or "dead length" occurring at the tip of the antenna. Attempts have been made to solve one or more of the above problems by providing implantable radiating antennas having improved performance characteristics.
For example, two-node and three-node microwave antennas have been proposed to expand the heating volume to as much as twice that provided by a single-node dipole antenna. However, such antennas have exhibited an inhomogenous heating pattern with three or four peaks along the antenna axis and a failure to heat effectively out of the antenna tip. A variable diameter dipole antenna has also been proposed to force the heating current into larger diameter sections of the antenna which fit snugly within the biocompatible plastic catheter. The larger diameter section at the tip of the antenna appears to provide more effective tip heating, but the antenna still exhibits considerable dependence of heating on insertion depth and periodic excessive surface tissue heating.
Other types of dipole antennas, such as the sleeved coaxial slot and balun-fed folded dipole, have been proposed for shifting the heating field out to the antenna tip. The concept of multiple breaks in the coax outer conductor of the antenna with each section being driven by a separate microwave source has held some promise for closer control of the depth heating profile, but at the expense of greatly increased equipment complexity. Although often accomplishing an expansion of the effective heating length for a given frequency and/or a reduction in dead length at the tip, the above types of antennas are commonly plagued with the same critical problem as that of the linearly polarized simple dipole antenna, namely, a critical dependence of the heating pattern on the depth of insertion.
Another proposed technique employs an "over-ride" reciprocated motion system for linearly translating the dipole antenna during treatment. Although this technique may potentially solve at least part of the axial heating pattern problem, predictability and real time control of the overall heating pattern would likely prove difficult due to power deposition pattern changes at different positions within the range of antenna movement.
A similar development of antenna designs has occurred for intracavitary heating applications. Antennas of this type having somewhat larger diameters (e.g., 1-1.5 cm v. 0.1-0.15 cm) have been used in the treatment of tissues surrounding body cavities. An antenna of the latter type has been constructed with a 1.0 cm diameter coax cable outer conductor cut in a helical manner and pulled apart axially to form a helical extenson of the antenna feedline outer conductor having ten turns extending 14 cm in length. Thermal profiles of the antenna were found to be quite variable for the different conditions studied and the antenna exhibited a strong dependence on both source frequency and insertion depth. Most tests were performed using insertion depths less than the 14 cm length coil.
A so-called "flexible leakage type" antenna has also been proposed for use at 2450 MHz. This type of antenna consists of a helical structure composed of 1.0 mm wide copper foil tape interconnected between the inner and outer conductors of a 2.0 mm diameter flexible coaxial cable.